Among the elements Cl, F, P, and S has the most negative electron affinity and which one has the least negative electron affinity?

Cl (chlorine) has the most negative electron affinity and P (phosphorus) has the least negative electron affinity.

Among all the elements, halogens have the most negative electron gain enthalpy because they require only one electron to complete their octet, so they readily accept the incoming electron. 

While F (fluorine) and Cl (chlorine) both are halogens, electron gain enthalpy is more negative for Cl because F has a smaller size due to which its electrons repel the incoming electron while in Cl, the electrons are dispersed due to its larger size than F, so the electron is easily added. 

P (phosphorus) has the least negative electron gain enthalpy because it has a larger size due to which its charge is less effective on the outer shell electrons.